Implantable cardiac devices typically sense cardiac electrical signals in an implantee and classify the implantee's cardiac rhythm as indicating therapy or not indicating therapy. A rhythm indicates therapy if it is believed that delivering therapy would provide an improvement to the patient's condition that outweighs discomfort or risk associated with therapy; otherwise, the rhythm does not indicate therapy. For example, ventricular fibrillation almost always indicates therapy, some ventricular tachyarrhythmia indicates therapy, and atrial fibrillation often may not indicate therapy. Clinical judgment as to whether therapy is indicated may vary.
The nomenclature used herein indicates that a signal is sensed by an implantable cardiac device, events are detected in the sensed signal (yielding detected events or detections), and cardiac activity is classified by use of the detected events in a rhythm classification step or process. Rhythm classification is often directed at identifying rhythms such as ventricular fibrillation or certain tachyarrhythmias indicating therapy. Some such systems then make therapy decisions reliant on the classification of the cardiac rhythm. Rhythm classification often includes interval or rate analysis. For example, detected events are separated by intervals, and several intervals can be used to generate an average interval. The detected heart rate can then be calculated using the average interval. Rate is often a primary factor in identifying conditions such as ventricular fibrillation (VF) and/or ventricular tachyarrhythmia.
To accomplish accurate rhythm classification using rate analysis, events should be detected accurately. A cardiac electrogram includes several portions (often referenced as “waves”) that, according to well known convention, are labeled with letters including P, Q, R, S, and T, each of which corresponds to particular physiological events. Detection methods are often designed to detect the R-wave or QRS complex, though this is not required. Typically for purposes of ascertaining rate, the aim is to predictably count each cardiac cycle, so any portion of the cardiac cycle, if repeatedly identified, can serve as the focus for detection. Overdetection (such as a double or triple detection) may occur if the device declares more detected events that its design contemplates for a single cardiac cycle. Examples of overdetection (in a system designed for a single detection per cardiac cycle) include the detection of both an R-wave and a trailing T-wave, multiple detections of a wide QRS complex, or early detection of a P-wave followed by detection of a trailing part of the QRS complex or a T-wave from the same cardiac cycle. Overdetection may also occur if noise causes an event to be declared when no cardiac event has taken place, for example, due to external defibrillation, external noise, pacing artifact, skeletal muscle noise, electro-therapy, etc. Even if rate analysis is not used in rhythm classification (for example, signal shape analysis could be used to the exclusion of rate), accurate detection of cardiac events would be useful to improve analytical accuracy.
Overdetection can lead to overcounting of cardiac cycles. For example, if one cardiac cycle takes place and a detection algorithm mis-identifies this one cycle as multiple cardiac cycles, overdetection has occurred. If the heart rate is then calculated by counting true detections as well as overdetections, overcounting occurs. Calculated heart rates may be used alone or in combination with other factors to classify cardiac rhythms. Miscalculation of heart rate can lead to incorrect rhythm classification and therapy decisions. For example, miscalculating heart rate by overcounting can cause elevated rate calculation and, since many implantable devices use rate for identifying conditions indicating therapy, for example ventricular fibrillation, miscalculating heart rate by overcounting can lead to inappropriate therapy. Inappropriate therapy is widely considered undesirable.
New and/or alternative methods and devices for cardiac signal analysis are desired.